This invention relates generally to remote sensors of the type used in earth orbiting satellites and, more specifically, to a method and apparatus for providing a geometrically corrected, large angle scan of a portion of the earth's surface using a linear array of radiation-sensitive detectors mounted in a satellite.
Earth orbiting satellites equipped with remote sensing apparatus are well-known. Satellites used to gather meteorological and geological information often carry one or more detectors which are sensitive to light or other forms of radiation. Mirrors, lenses and other optical components are typically used to focus and direct incident radiation to the detector(s). The output(s) of the detector(s) may be connected to signal processing apparatus which processes the data received from the detector(s) and transmits it (either directly or following an intervening storage operation) back to earth.
A number of techniques for scanning the earth's surface with one or more radiation sensitive detectors are known. For example, a single detector may be rotated around an axis which extends coaxially with the velocity vector of the satellite. This technique produces a series of left-to-right (or right-to-left, depending upon the direction of rotation) scans across the earth's surface. If the rotation rate is high, relative to the velocity of the satellite, the left-to-right scans are generally perpendicular to the direction of travel. If the rate of rotation is slow, the scans will be nonperpendicular or skewed in the direction of travel. The effect of spacecraft velocity on scan geometry is discussed more fully below in connection with the subject invention.
It should be noted that, for purposes of this discussion, references to rotation or other movements of a single detector or an array of detectors for purposes of scanning the earth's surface may refer to actual movement of the detector relative to the earth's surface or other frame of reference or, alternatively, to rotation or movement of a mirror or other optical device relative to a "stationary" detector or detector array. Indeed, use of a rotating or reciprocating mirror to direct radiation to a detector or detector array is generally preferred.
Use of a single rotating detector offers advantages of straightforward design and simplicity in operation and maintenance. However, resolution may be somewhat limited, especially when a relatively high rotation velocity is required. A rotating detector is also somewhat inefficient since, depending upon the scan angle designated, the detector will be facing away from the target surface for a large part of each revolution.
An alternative to the single detector approach involves use of a linear array of detectors. One way in which a linear array of detectors may be used which does not involve mechanical scanning (i.e., movement of the array) is known as "push broom" scanning. In a system of this type, a stationary linear array of detectors is positioned perpendicularly to the velocity vector of the craft, and facing the surface to be scanned. This approach has been successfully implemented in applications requiring relatively small scan angles. In a push broom scanning system, the optical throughput is relatively high, the electrical bandwidth is relatively low, and there are fewer moving parts than in a mechanical scanning system. However, there are at least three significant problems which preclude the use of a push broom scanner in applications which specify relative large scan angles. One problem is unacceptable aberrations associated with very wide angle optics required to generate the desired scan swath. A second problem involves difficulty in achieving true radiometric calibration of the push broom device. A third problem relates to the relative electronic complexity associated with the large number of detector elements which must be used in order to achieve an acceptable resolution across a relatively wide swath.
A mechanical scanning technique in which a linear array of detectors may be advantageously used to obtain a relatively large angle scan is known as the "whiskbroom" scanning technique. In a whiskbroom scanner, a linear array of detectors is aligned in parallel with the velocity vector of the satellite (i.e., the along-track direction) and moved from side to side in a reciprocating manner generally perpendicularly to the velocity vector of the craft (i.e., the along-scan direction). The benefit of whiskbroom scanning, as compared to the single detector approach, can be described in terms of longer dwell or integration times which result in increases in resolution and greater signal-to-noise ratios. There is also a potential for achieving much greater scan efficiency with this approach since the detectors in the array are pointed toward the "target" for a much greater percentage of the available scanning time. However, certain geometrical anomalies peculiar to the whiskbroom approach can seriously degrade the image produced by a large angle scan by causing an overlapping or interweaving of contiguous scans. This degradation may be worsened by distortion caused by the velocity of the spacecraft unless a relatively complex mechanical arrangement is used to compensate for velocity in the scanning apparatus.
An object of the present invention is to provide a large angle whiskbroom scanning arrangement which offers improvements over other scanning arrangements known in the prior art.
Another object of the present invention is to provide a large angle whiskbroom scanning arrangement in which the geometrical anomalies inherent in the large angle whiskbroom scanner are addressed and accounted for advantageously.
Yet another object of the present invention is to provide a large angle whiskbroom scanning arrangement which compensates for distortion caused by the forward velocity of the spacecraft.
These and other objects are attained by a scanning arrangement for providing a geometrically corrected large angle whiskbroom scan of a portion of the earth's surface using a linear array of radiation-sensitive detectors mounted in a low earth orbiting satellite. The invention may be described in terms of a method which includes the following steps: scanning a portion of the earth's surface with the array of detectors to define an irregularly-shaped swath on the surface which represents a field of view of the array; defining a regularly-shaped ground area of interest within the field of view of the array; dividing the ground area of interest into a plurality of pixel lines extending generally perpendicularly to the array, the number of pixel lines being fewer in number than the number of detectors in the array; subdividing each pixel line into a plurality of elements, each element encompassing at least a portion of a field of view of one or more of the detectors in the array; selectively combining outputs from the detectors to provide a data signal representative of each of the individual elements in each pixel line; and reconstructing a representation of the ground area of interest from the data signals. In a preferred embodiment of the invention, the ground area of interest is rectangularly-shaped and extends in the along-scan direction generally perpendicularly from the linear array. In this embodiment, the ratio of detectors in the array to pixel lines is at least three-to-one, and the in-scan detector subtense corresponds to a distance of approximately 500-600 meters along the ground. The step of selectively combining outputs from one or more of the detectors to provide a data signal representative of each of the individual elements in each pixel line includes normalizing the combined outputs of the detectors to provide for constant resolution across the scan. The outputs from the detectors are combined, and the representation of the ground area of interest is reconstructed, in accordance with a preferred set of conditions and relationships.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.